1. Field of the Invention
The present invention is directed generally to Quantum Well Infrared photodetector Focal Plane Arrays (QWIP FPA""s) and, more particularly, to QWIP FPA""s that are capable of discriminating hot gas infrared sources from solar and thermal xe2x80x9cclutter.xe2x80x9d
2. State of the Art
Accurate detection of infrared sources from solar and thermal infrared xe2x80x9cclutterxe2x80x9d is a common problem in a number of applications, including the detection of vehicle or aircraft exhaust, smoke stacks or other hot gas sources. In vehicle detection systems, a vehicle such as an airborne object (e.g., a rocket) can be identified by the hot gas exhaust emissions from the vehicle""s engine. A vehicle engine such as a rocket engine can generally be modeled as sublimating carbon dioxide. Water vapor and carbon dioxide present in the earth""s atmosphere produces a deep notch in the middle of the mid infrared atmospheric window. The infrared signature of a rocket engine as viewed from a distance through the earth""s atmosphere thus has a characteristic blue impulse and a moderate red response. The moderated red response is due to spreading in the atmosphere and cooling effects from the hot gas exhaust of the engine.
The infrared signature of a rocket engine is generally bright and relatively easily detected. However, in certain environments, such as an urban environment, there are many false infrared sources. The primary two false infrared sources are solar and thermal xe2x80x9cclutter.xe2x80x9d Solar clutter arises from sun glint reflected off of highly reflective surfaces and thermal clutter arises from thermal self emission from relatively warm objects. In early warning systems, solar and thermal clutter limits detection performance and increases false alarm rates. Solar reflectance causes the highest false alarm rates as the equivalent black body model for the sun acts in a manner very similar to a rocket engine. Additionally, fast moving objects such as the exhaust from large trucks, the warm engines of small aircraft, as well as emission from black tar and roof tops may also act to cause false alarms in a detection system. Solar and thermal xe2x80x9cclutterxe2x80x9d therefore make discrimination of a rocket engine difficult in certain environments.
One conventional approach to an early warning system that attempts to discriminate the rocket engine from xe2x80x9cclutterxe2x80x9d uses a single wave length system. In this conventional system, either the red or blue windows of the mid-band are selected for detection. However, with only one of these windows selected, a rocket launch, sun glint, or a warm black body may all produce a similar response. Thus, in this conventional system, the only way to detect differences between the rocket plume and clutter is to discriminate on the basis of parameters such as size, velocity, or duration.
Another conventional approach is to add an additional wavelength to produce a two color detection system. This system can thus detect the red response and the blue response band simultaneously or within the same video frame. A comparison between the system output signals can determine if the detected flux is solar or thermal xe2x80x9cclutterxe2x80x9d or if the detected flux originates from a rocket engine. If a comparison indicates that the blue response is greater than the red, then solar xe2x80x9cclutterxe2x80x9d is indicated. If the red response equals the blue response, then thermal xe2x80x9cclutterxe2x80x9d is indicated. If the red response is greater then the blue response, then a missile is indicated. This two color system is an improvement over the single wavelength system but relies on complicated algorithms, precise filters, and stable detectors to provide the output comparison. One example of a two color system is a conventional mechanical color wheel which uses a wide-band IR detector and an associated rotating mechanical multicolor filter wheel. The wide-band IR detector detects a broad range of incident wavelengths and the rotating filter wheel selects the desired wavelength that is to be passed to the wide-band detector. An exemplary color wheel system is disclosed in U.S. Pat. No. 5,300,780. Mechanical color wheel systems, however, suffer from a number of deficiencies in multicolor detection. Such systems generally are slow and bulky, require large amounts of power for operation, and have a limited life span. Additionally, color wheel systems tend to have poor photon collection efficiency.
Thus, it would be advantageous to construct a multicolor detection system that can discriminate hot gas sources such as, for example, rocket engines, from solar and thermal xe2x80x9cclutter.xe2x80x9d
Discrimination of hot gas sources from solar and thermal clutter is achieved in exemplary embodiments of the invention using a multicolor detection system that can include a photodetector focal plane array. In exemplary embodiments, a detector structure of the array comprises two vertically stacked quantum well layers. Each of the quantum well layers are individually biased by separate bias voltages and the separate bias voltages are modulated to produce two or more measurements at different spectral bands over a given sampling interval. Each detector structure of the array can thus perform measurements of incident infrared energy in at least four separate spectral bands. This technique of measuring incident infrared energy in four separate spectral bands can advantageously be applied to the discrimination of hot gas sources from background infrared clutter.
One exemplary embodiment of the present invention is directed to a photosensitive device comprising: a first photosensitive layer; a second photosensitive layer, wherein said photosensitive layers are formed adjacent one another; and means for applying a plurality of bias voltages across each of said layers over a sampling interval to vary a spectral responsivity associated with each layer.
An additional exemplary embodiment of the present invention is directed to a method of detecting the presence of a remote hot gas infrared source comprising the steps of: measuring a first infrared flux at a first wavelength; measuring a second infrared flux at a second wavelength, where said first wavelength is greater than said second wavelength; measuring a third infrared flux at a third wavelength, where said third wavelength is greater than said first wavelength; measuring a fourth infrared flux at a fourth wavelength, where said fourth wavelength is greater than said third wavelength; indicating the presence of said remote hot gas source if said second infrared flux is less than said first infrared flux and said fourth infrared flux is less than said third infrared flux.
A further exemplary embodiment of the present invention is directed to a quantum well focal plane array comprising: means for receiving incident infrared radiation, wherein said incident radiation is comprised of radiation from a hot gas source and radiation from infrared clutter; means for converting said received incident infrared radiation into moving charges; and means for processing said moving charges to discriminate said hot gas source infrared radiation from said infrared clutter radiation.
An exemplary method of biasing an infrared photodetector of the present invention comprises the steps of: a) separately biasing first and second portions of a detector structure such that said first portion is responsive to a first spectral band and said second portion is responsive to a second spectral band; and b) separately biasing said first and second portions of a detector structure such that said first portion is responsive to a third spectral band and said second portion is responsive to a fourth spectral band.
An exemplary method of discriminating an infrared target from infrared clutter comprises the steps of: receiving radiation in first and second portions of a detector structure, wherein a first quantity of said received radiation is associated with said infrared clutter and a second quantity of said received radiation is associated with said infrared target; applying a first voltage to said first portion of said detector structure to produce a first quantity of moving charges responsive to said received radiation; applying a second voltage bias to said second portion of said detector structure to produce a second quantity of moving charges responsive to said received radiation; varying said first voltage bias to produce a third quantity of moving charges responsive to said received radiation; varying said second voltage bias to produce a fourth quantity of moving charges responsive to said received radiation; and comparing voltages derived from each of said first, second, third, and fourth quantities of moving charges to discriminate said infrared target from said infrared clutter.